The rapid growth of the "information highway" has created the need for high-speed, low-cost techniques for transmitting data to and from homes, small businesses, schools and the like. At the data rates of conventional modems, the transmission of detailed graphics, for example, typically requires a time that may be annoying to the user. A web page containing detailed graphics of 100 kilobytes may require 27 seconds for transmission. Optical fiber networks and CATV networks have sufficient bandwidth to permit high speed data transmission. However, the infrastructure is not presently available to provide data services to consumers on optical fiber or CATV networks on a widespread basis and at low cost.
An ADSL standard for data transmission is being developed to address these issues. Data transmission, according to the ADSL standard, permits transmission of simplex and duplex digital data signals over the conventional twisted wire pairs that are used for plain old telephone service (POTS). The digital data signals are transmitted at frequencies above the baseband analog POTS band (0-4 kilohertz). The ADSL standard is a physical layer standard providing for a simplex downstream channel at a maximum rate of 6.2 megabits per second and a minimum rate of 1.544 megabits per second. The ADSL standard also includes a duplex digital channel at optional rates of 64 kilobits per second, 160 kilobits per second, 384 kilobits per second and 576 kilobits per second. The ADSL standard takes advantage of the fact that most consumer applications, such as Internet access, access to online information services, access to private networks and work-at-home applications, require a larger bandwidth into the home than out of the home. ADSL transport technology is described by R. Olshansky in "Moving Toward Low-Cost Access to the Information Highway", Telephony, Nov. 7, 1994, pp. 31-37.
The basic ADSL architecture includes an ADSL interface unit at the telephone company central office (CO) and an ADSL interface unit at the customer location, interconnected by a twisted pair of conductors. Each ADSL interface unit includes a POTS splitter and ADSL modem. The ADSL modem transmits and receives digital data on the twisted pair at the selected ADSL transmission rates. At the central office, the ADSL interface unit receives and transmits digital data from a wide area network. The POTS splitter frequency multiplexes the high speed digital data and the analog POTS signal onto the twisted pair for transmission. The POTS splitter at the customer location decouples the analog POTS signal onto the existing POTS wiring in the home. The ADSL modem receives the ADSL data signals and forwards them to the customers PC or LAN. The interface between the LAN and the ADSL channel must be configured to insure efficient transfer of digital data. Similarly, the interface between the wide area network and the ADSL channel at the CO must be configured to insure efficient transfer of digital data.